There is often a need as for example in various manufacturing processes to be able to measure viscosity in a continuous mode and with a low level of error, for example to establish the viscous and elastic properties of visco-elastic substances. However, in such a situation problems frequently arise by virtue of the fact that the material to be measured is frequently under an elevated pressure and in many cases is also at elevated temperature. In one form of such a viscosimeter, the device comprises a housing which can be fitted to a space or measuring chamber containing the substance in respect of which viscosity measurements are to be taken, the housing defining an internal chamber and having a connecting rod system which is mounted in the housing and which extends into the measuring chamber. A metal bellows member surrounds the connecting system in the measuring chamber and has one end secured to the outside of the wall of the housing. The viscosimeter further includes a measuring system comprising a rotationally symmetrical measuring member at the end of the connecting system which projects into the measuring chamber, together with a drive means including a torque measuring means operatively connected to the connecting system to produce a measuring flow as between the measuring member and the material in the measuring chamber. It will thus be appreciated that, having regard to the above-mentioned general likelihood of the material to be measured being under elevated pressure and possibly also at elevated temperature, it is necessary for the connecting system to be carefully sealed off with respect to the torque measuring means. As a further consideration in that respect, the sealing means should, as far as possible, not give rise to any force or torque losses, as otherwise that would result in measuring errors.
In one particular design of such a viscosimeter, as disclosed in German patent specification No. 2 632 076, the measuring member rotates continuously and it is only the viscous properties of the material to be measured that it determines. The rotary movement is transmitted in a friction-free manner into the measurement chamber by way of a magnetic coupling means. However, angular displacement may occur as between the two halves of the magnetic coupling means, in dependence on the torque produced. It will be appreciated that it would be difficult accurately to oversee and assess such angular displacement. Furthermore, while such angular displacement is not a major problem in the case of a viscosimeter which operates by virtue of a continuous rotary movement of the measuring member, it is however a problem in the case of an oscillatory viscosimeter of the general kind just discussed above. Thus, in such an oscillatory viscosimeter, there must be a clear association between the angular phase position of the driven measuring member which is disposed in the measuring chamber, and the position of the drive means for driving same. Desirably, the degrees of displacement should be identical under all circumstances.
In another design of viscosimeter, as disclosed in German patent specification No. 2 330 964, the measuring member also rotates continuously, being driven by a hollow shaft which extends into the measuring chamber. The degree of twist of the shaft constitutes a measurement in respect of the torque value involved. However, it will be noted that the hollow shaft which thus serves at the same time as the torque measuring means is exposed to the thermal, chemical and abrasive effects of the material being measured, while in addition the point at which it passes into the measuring chamber must be suitably sealed by means of a gland or like sealing arrangement. That can give rise to serious difficulties, having regard to the above indicated properties of the material to be measured.
Other oscillatory viscosimeters which provide for continuous rotation are also known, as disclosed for example in German patent specification No. 2 006 119 and U.S. Pat. No. 2,683,984 to Boyle et al, wherein the measuring shaft is enclosed by a metal bellows and in that way is sealed off, without however giving rise to a frictional effect. The longitudinal axis of the metal bellows in that arrangement extends approximately parallel to the axis of rotation of the system. However, in those designs, the axis of rotation does not coincide with the geometrical axis of the measuring member so that the measuring member performs a wobble movement as it rotates. As a consequence, the measuring flow which is produced in that way cannot be ascertained mathematically, and the arrangement does not produce defined shear conditions insofar as, with the wobble motion, what is produced is not a straightforward parallel or stratified flow which can be used as a basis for computation, but a displacement flow which evades precise computation.
Another form of oscillatory viscosimeter, as disclosed in German laid-open application (DE-OS) No. 33 24 842, comprises an outer cylinder which performs forced rotary oscillatory movements, and an inner cylinder which is coaxial with respect to the outer cylinder and which is fixed to a hollow shaft or to a torsion tube. The degree of twist of the torsion tube which extends into the material in respect of which a measurement is to be taken is measured, such twist in turn depending on the torque involved in the system. A pump urges the material to be measured through the annular gap or clearance between the inner and outer cylinders. That design of viscosimeter suffers from the disadvantage that either the outer cylinder must have a sealing element which is exposed to the material in respect of which measurements are to be taken, or, if no sealing means is provided, some of the material to be measured is lost. The torsion tube which forms the torque measuring means is also exposed to the effect of the material in respect of which measurements are to be taken, while in addition that viscosimeter can only be used in a secondary flow in by-pass relationship to the actual process flow, and not in the process flow itself.